Flexographic printing is widely used in the production of newspapers and in the decorative or functional printing of packaging media. Photosensitive printing plates have been developed to meet the demand for fast, inexpensive processing and long press runs.
The photosensitive printing element generally comprises a support layer, one or more photosensitive layers, a slip film release layer, and a protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. A slip film may be disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and/or act as an ink-accepting layer.
The photosensitive resin composition is deposited onto the support layer to later form a printing element. The photosensitive resin composition can be deposited onto the substrate in a variety of ways, e.g., by extrusion, roll coating, heat processing, solvent casting, and the like. These techniques can be readily carried out by those skilled in the art.
The desired image is produced on the printing plate by exposing selected portions of the photosensitive resin to actinic radiation, which is traditionally accomplished in one of three related ways. In the first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer, which is also sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative, and the printing element is then flood exposed through the in situ negative. In the third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods produces an acceptable result, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer. The unexposed and therefore unhardened portions of the resin are then developed.
The support sheet, or backing layer, can be formed from a suitable transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred substrate materials include steel, copper or aluminum sheets, plates, or foils, paper, or films or sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like.
The photosensitive layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a solid composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzrnacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If a second photocurable layer is used, i.e., an overcoat layer, it typically is disposed upon the first layer and is similar in composition.
The photosensitive materials generally cross-link (cure) and harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV wavelength regions. Preferred actinic wavelength regions are from about 250 nm to about 450 nm, more preferably from about 300 nm to about 400 nm, even more preferably from about 320 nm to about 380 nm. One suitable source of actinic radiation is a UV lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin sheet, which protects the photosensitive layer from dust and increases its ease of handling. In a conventional plate making process, the slip film is substantially transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. The slip film typically comprises a polymer resin that is compatible with the underlying photopolymer of the photosensitive layer and is easily removable during the development step. The polymer resin may be selected from the group consisting of polyacetals, polyacrylics, polyamides, polyimides, polybutylenes, polycarbonates, polyesters, polyethylenes, cellulosic polymers, polyphenylene ethers, polyethylene oxides, and combinations of the foregoing. Preferred polymer resins include polyamides, and cellulosic polymers, such as hydroxypropyl cellulose.
It is often desirable to use a slip film layer in a photosensitive printing element because the surface of the photosensitive composition tends to be sticky and, when an original film is applied thereon, bubbles are formed between the composition surface and the original film. When the photosensitive printing element is then exposed to actinic radiation, irregular deflection of light may occur, hindering the light exposure and curing of the photosensitive layer. Consequently, the reproducibility of the relief is deteriorated and the original film applied on the photosensitive layer becomes difficult to reuse. By adhering the slip film to the photosensitive resin layer, the adhesion of the original film to the photosensitive resin composition surface is mitigated and the original film can be reused.
In “digital” plate making processes, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative on a digital (i.e., laser ablatable) masking layer, which is generally a modified slip film. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. The laser ablatable layer can be any photoablative masking layer known in the art. Examples of such laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. The laser ablatable layer generally comprises a radiation absorbing compound and a binder. The radiation absorbing compound is chosen to be sensitive to the wavelength of the laser and is generally selected from dark inorganic pigments, carbon black, and graphite.
After imaging, the photosensitive printing element is developed to remove the masking layer and the unpolymerized portions of the layer of photocurable material to create a relief image on the surface of the photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter
Washing may be accomplished by a variety of processes, including brushing, spraying, or immersion. The resulting surface has a relief pattern that reproduces the image to be printed. The printing element is mounted on a press and printing commences.
In the case of heat or thermal development, the composition of the photopolymer is such that there exists a substantial difference in the melt temperature between the cured and uncured polymer to allow the creation of an image in the photopolymer when heated. Photocurable compositions that are thermally developable are described, for example, in U.S. patent application Ser. No. 10/353,446 filed Jan. 29, 2003, the teachings of which are incorporated herein by reference in their entirety. Other suitable compositions would also be known to those skilled in the art. The difference in melt temperature allows the uncured photopolymer to be selectively removed thereby creating an image. The uncured photopolymer (i.e., the portions of the photopolymer not contacted with actinic radiation) melts or substantially softens while the cured photopolymer will remain solid and intact at the temperature chosen. Then, uncured photopolymer can be melted or removed by contacting the heated printing element with a material that will absorb or otherwise remove the softened or melted uncured photopolymer (i.e., “blotting”).
While many slip film compositions are described in the prior art, there remains a need in the art for further improvements to the slip films in the manufacture of flexographic printing plates to improve various processing characteristics of the printing plate.
The inventors have surprisingly discovered that the addition of a layered silicate material as a filler, more preferably a montmorillonite clay, into a slip film composition, produces a slip film composition with improved release properties for negative artwork and improved thermal processing characteristics as compared to slip film compositions of the prior art.
U.S. Pat. No. 5,039,592 to Umeda, the subject matter of which is herein incorporated by reference in its entirety describes a slip film composition that contains a dyestuff, especially a photobleaching dyestuff to control the amount of incident light and reflected light from the support at a relatively upper portion of the photosensitive layer and give a better quality image at a shadow area of the printing plate. However, Umeda do not describe any improvements in either the release properties of the slip film or the thermal processing characteristics of the slip film.
U.S. Pat. No. 5,506,086 to Van Zoeren, the subject matter of which is herein incorporated by reference in its entirety, describes a material capture sheet having one or more layers which assist in the capture of debris resulting from laser irradiation of an infrared sensitive layer. Optionally, the layer or layers on the material capture sheet can include a matte agent to create a separation gap between the infrared sensitive layer and the material capture sheet.
U.S. Pat. No. 6,410,635 to Kaylo et al., the subject matter of which is herein incorporated by reference in its entirety, generally describes compositions comprising an exfoliated silicate material (i.e., a montmorillonite clay) to enhance properties such as appearance, crater resistance, and rheology control. Kaylo et al. suggest that the compositions are particularly useful as color and/or clear coatings in color-clear composite coatings. However, Kaylo et al. do not suggest that the compositions are usable as slip film compositions for a flexographic printing plate.